This invention relates to a brake drum particularly adapted for motor vehicles and being reinforced with an embedded reinforcement member to provide a composite structure.
Brake drums used for motor vehicles such as heavy duty trucks are typically formed by casting grey iron and machining the casting in areas where precision dimensions and surfaces are required. Although iron brake drums perform satisfactorily, designers of braking systems are constantly striving for enhanced performance, lower cost, increased fatigue life and lighter weight. One particular shortcoming of conventional all-iron drums is their susceptibility to heat checking and crack formation which can lead to ultimate failure of the drum.
As a means for providing an improved brake drum, composite structures are known according to the prior art in which steel is incorporated into an iron brake drum reinforcement. For example, in accordance with U.S. Pat. No. 2,316,029, a bell-shaped stamped sheet metal housing is provided having an iron inner portion forming the friction surface of the drum which is centrifugally cast in place. Although drums of this construction operate satisfactorily, the location of the steel reinforcing layer is not optimized since the higher bending stresses imposed on the brake drum by the brake shoes are very close to the inside cylindrical surfaces of the braking surface where the reinforcement of steel can be most advantageously used. Moreover, the process of manufacture of such a drum would require specialized machinery and processing steps. Another approach used in the past is to provide an externally applied reinforcing member such as a steel band as taught by U.S. Pat. No. 3,841,448. This approach also requires specialized fabrication equipment and further does not optimally locate the steel reinforcing member. Moreover, the interface surfaces between the drum and reinforcement need to be precision machined and providing a good bond between the parts can be difficult. A steel wire ring is embedded within an iron brake drum structure according to U.S. Pat. No. 2,111,709. Although this structure would likely provide improvements over an all-iron brake drum according to the prior art, the reinforcement provided by the single ring is positioned only to reinforce the open mouth of the brake drum. In additional, no means for positioning the reinforcing member during the molding process is disclosed by this patent. The large cross-sectional area of a single reinforcing ring could further lead to poor bonding between the iron and steel ring due to the heat sink imposed by the ring.
In accordance with this invention, an improved composite brake drum is provided which achieves a number of significant benefits over prior art cast brake drums. The brake drum according to this invention employs a cage-like reinforcement assembly preferably made from steel wire which is cast in place to be substantially embedded within a grey iron brake drum. Locating means are provided to enable the reinforcement structure to be accurately positioned with respect to the mold cavity during casting. Since the steel material of the reinforcing assembly has a considerably higher modulus of elasticity then grey iron, the reinforcement increases the strength of the composite drum structure, thus decreasing mechanical deflection in response to loading. This reinforcement also reduces the generation of surface checks and cracks which can propagate to ultimately cause mechanical failure of the brake drum. The increased strength of the composite further enables a decrease quantity of iron necessary to provide a given strength brake drum, thus resulting in a lighter weight brake drum structure. The reinforcement assembly further locates reinforcing sections close to the friction surfaces of the drum where it is most advantageously positioned for structural efficiency. The axial extent of the reinforcing member serves to reinforce the brake drum across the entire depth of the friction braking surface. Significantly, the composite brake drum according to this invention can be fabricated using conventional sand casting processes with minimal variations, thus saving the cost of retooling. Due to the fact that the metal reinforcing sections according to the invention are distributed, relatively small diameters of wire can be used which enables the wire to be rapidly heated to near the temperature of the molten iron being poured into the casting mold, thus producing good fusion between the iron and embedded steel reinforcement.
Additional benefits and advantages of the present invention will become apparent to those skilled in the art to which this invention relates from the subsequent description of the preferred embodiments and the appended claims, taken in conjunction with the accompanying drawings.